I've been looking at the 6502 processor because it is cheap but then people wouldn't buy a computer unless it could get on the internet or play youtube videos. I'm wondering what other processors are available to build a computer from.

There are probably over 100 different manufacturers of microprocessors, each with 100 or more different chips.

EDN magazine publishes a "Microprocessor Directory" each year. Or did anyway. The most recent one I could find has dead links, but is still an interesting list of manufacturers: http://www.edn-europe.com/microdirectory.asp

Many modern microprocessors are not very friendly to the hobbyist or small scale manufacturing. That probably applies to most of the micros capable of connecting to the internet and playing video. Sometimes there are cheap, small, "development boards" that can be used if your main interest is software development or adding small amounts of hardware. "Chumby" and "BeagleBoard" are examples in this class.

For a interesting and philosophical look at some of the microprocessors around that ARE accessible to the hobbyist, check out my Instructable:http://www.instructables.com/id/How-to-choose-a-MicroController/

if you are looking for net and video its going to be hard outside of what is already standard, I picked up a dual core 1.6ghz atom micro atx with everything but ram on board for 60 bucks shipped on ebay, it runs windows 7 x64, and has meh performance with 720 video full screen

add ram and maybe a pico power supply and you can run a balls out netbook for ~100 US$ off of a car battery in a large cigar box for a month

Yeah; the instructable specifically mentions what I call "The $100 paradox." You can build your own computer from scratch; make (or have made) a PCB, buy individual chips, etc. It'll probably end up costing you about $100 for the pieces and equipment and stuff.Or you can get some commercial stuff that is re-purposeable into a general purpose computer. It'll probably cost about $100. (Chumby, for example.)Or you can buy one of the more extensive existing development boards (hawkboard, beagleboard, etc.) They're about $100.Or you get get a low-end palmtop, or a used last-generation desktop. They'll probably run about $100.

Which of those is your best option depends on exactly what you want to do. It's VERY difficult to beat the "value" of a used desktop computer (frequently available for FREE.) But they won't fit in your autonomous air vehicle...

One reason that Arduino is so popular is that it's only about $30. (Though if you add a motor driver and and xxx shield and some other stuff to bring it up closer to the more complex development boards, it'll likely cost ... about $100.)